1. Field of the Invention
The present invention relates to touch panels, which are mainly used to control various electronic devices.
2. Background Art
In recent years, various electronic devices such as mobile phones and car navigation systems are becoming increasingly functional and diverse. In line with this trend, more and more devices have alight transmissive touch panel attached to the front surface of a display element. Such a touch panel allows the user to select functions shown on the display behind the touch panel by pressing on the panel with a finger or a pen. Therefore, touch panels are required to provide excellent visibility and reliable operation.
A conventional touch panel is described with reference to FIGS. 4 and 5. These drawings are shown in an enlarged scale in the thickness direction in order to make the structure more understandable. FIG. 4 is a sectional view of an essential part of the conventional touch panel, and FIG. 5 is a plan view of the touch panel.
Conventional touch panel 40 includes top substrate 1 and bottom substrate 2, which is disposed under top substrate 1. Top substrate 1 is a light transmissive film, and bottom substrate 2 is also light transmissive. An upper conductive layer 3, which is made of a light transmissive material such as indium tin oxide, is provided on the bottom surface of the top substrate 1. A lower conductive layer 4, which is made of the same material as upper conductive layer 3, is provided on the top surface of the bottom substrate 2.
The top surface of lower conductive layer 4 has a plurality of dot spacers (unillustrated) made of an insulating resin and formed at predetermined intervals thereon. Upper conductive layer 3 has a pair of upper electrodes 5 at both ends thereof in the width direction (for example, in the right and left direction in FIG. 5). Lower conductive layer 4 has a pair of lower electrodes 6 which are at both ends thereof in the width direction and perpendicular to upper electrodes 5 (in the top and bottom direction in FIG. 5). Upper electrodes 5 and lower electrodes 6 are made of a conductive material such as silver.
The bottom surface of top substrate 1 or the top surface of bottom substrate 2 is provided at the periphery thereof with frame-like spacer 7. Spacer 7 has an adhesive layer (unillustrated) applied to the top and bottom sides thereof, so that the periphery of top substrate 1 and the periphery of bottom substrate 2 are bonded together with spacer 7. With top substrate 1 and bottom substrate 2 bonded to each other, upper conductive layer 3 and lower conductive layer 4 are opposed to each other with a predetermined gap therebetween. The end of upper electrodes 5 and the end of lower electrodes 6 are protruded from one end of top substrate 1 and bottom substrate 2.
Touch panel 40 further includes film-type wiring board 8 having slit 8A at one end thereof. Wiring board 8 has a plurality of upper wiring patterns 9 on one surface (the top surface in FIGS. 4 and 5) and a plurality of lower wiring patterns 10 on the other surface (the bottom surface in FIGS. 4 and 5). Upper wiring patterns 9 and lower wiring patterns 10 are exposed at both ends of wiring board 8 so as to be connected to other elements. The top and bottom surfaces of wiring board 8 excluding the end portions are provided with an insulating layer (unillustrated) to coat the upper and lower wiring patterns. One end of wiring board 8 is sandwiched between top substrate 1 and bottom substrate 2. Upper wiring patterns 9 and lower wiring patterns 10 are made of a conductive material such as silver.
The aforementioned end of wiring board 8 is fixedly bonded between top substrate 1 and bottom substrate 2 by using anisotropic conductive adhesive 11. Anisotropic conductive adhesive 11 includes synthetic resin 11A and conductive particles 11B dispersed in synthetic resin 11A. Touch panel 40 is constructed in this manner by electrically connecting the end of upper electrodes 5 to one end of upper wiring patterns 9 and also connecting the end of lower electrodes 6 to one end of lower wiring patterns 10 via conductive particles 11B.
Touch panel 40 thus structured is attached to the front surface of a liquid crystal display or the like of an electronic device. The other end of upper wiring patterns 9 of wiring board 8 and the other end of lower wiring patterns 10 are connected to an electronic circuit (unillustrated) of the device by a connector or soldering.
Conventional touch panel 40 operates as follows.
The user presses on the top surface of top substrate 1 with a finger, a pen, or the like while watching the display of the liquid crystal display behind touch panel 40. As a result, top substrate 1 is bent downward to bring the pressed portion of upper conductive layer 3 into contact with lower conductive layer 4.
At the same time, the unillustrated electronic circuit supplies a voltage sequentially to upper electrodes 5 and lower electrodes 6 via upper wiring patterns 9 and lower wiring patterns 10, respectively, of wiring board 8. When upper conductive layer 3 is brought into contact with lower conductive layer 4, the electronic circuit detects the point of contact from the voltage ratio between upper and lower electrodes. Thus, the electronic circuit can switch various functions corresponding to pressed points.
Touch panel 40 is manufactured as follows.
First of all, top substrate 1 and bottom substrate 2, which have nearly the same size, are bonded together by spacer 7 disposed therebetween. At this moment, one end of wiring board 8 is sandwiched between the portion of top substrate 1 where the end of upper electrodes 5 is protruded and the portion of bottom substrate 2 where the end of lower electrodes 6 is protruded. The positioning is performed in such a manner that upper wiring patterns 9 are overlapped with upper electrodes 5 and lower wiring patterns 10 are overlapped with lower electrodes 6.
Then, the sandwiched end of wiring board 8 is heat-pressed at a predetermined temperature from both the top surface of top substrate 1 and the bottom surface of bottom substrate 2 by using a tool or the like. The heat pressing hardens anisotropic conductive adhesive 11 applied to the bottom surface of top substrate 1, the top surface of bottom substrate 2, or the end of wiring board 8. Hardened anisotropic conductive adhesive 11 allows the end of upper electrodes 5 to be bonded and electrically connected to the end of upper wiring patterns 9, and the end of lower electrodes 6 to be bonded and electrically connected to the end of lower wiring patterns 10. Touch panel 40 connected to wiring board 8 is manufactured in this manner.
The bond and connection between wiring board 8 and top and bottom substrates 1 and 2 by using anisotropic conductive adhesive 11 is needed to be performed while maintaining the predetermined temperature. When the heat press is performed at a temperature lower than the predetermined temperature, anisotropic conductive adhesive 11 applied between wiring board 8 and top substrate 1 or bottom substrate 2 is insufficiently hardened. The insufficient hardening makes the bond strength of wiring board 8 low, thereby causing unstable electrical connection between upper electrodes 5 and upper wiring patterns 9 and between lower electrodes 6 and lower wiring patterns 10.
Therefore, it is necessary to check the heating temperature before and after the connection process by the heat press. It is also necessary after the connection process to check the connection condition of wiring board 8 by examining the resistance value between the upper/lower electrodes and the upper/lower wiring patterns or the adhesion condition using a microscope or the like. One such conventional art is disclosed in Japanese Patent Unexamined Publication No. 2003-58319.
In this conventional touch panel, however, it takes time to check the heating temperature and the adhesion condition when wiring board 8 is bonded and connected to the top and bottom substrates. Furthermore, if there is a variation in the resistance value or the adhesion condition, it is difficult and takes time to determine whether the resistance value and the adhesion condition are sufficient or not.